THE NAUTILUS. 



VOL. ix. FEBRUARY, 1896. No. 10 



THE ATJLACOPODA: A PRIMARY DIVISION OF THE MONOTREMATE 

 LAND PULMONATA. 



BY HENRY A. PILSBRY. 



If the foot of a living or properly preserved Helix or Polygyra 

 be compared with that of a Zonites, Pyramidula (Patula) or Arlo- 

 Umax, a conspicuous point of difference becomes apparent. In the 

 latter three genera a groove or furrow may be seen running the 

 whole length of the foot, a short distance above and parallel to its 

 edges ; while no such structure is present in the first named genera. 



These furrows are the pedal grooves. They rise slightly at the 

 tail, and meet above it ; and at this point there is often a local 

 deepening of the groove, and a greater development of the mucus- 

 secreting cells lining it. This is the " caudal mucus pore," conspic- 

 uous in Zonites, Ariofi/iia.r, etc. There is vast variation in the 

 degree of development of this pore, from a barely visible widening 

 of the united pedal grooves, to a deep slit or a triangular pit sur- 

 mounted by a fleshy horn ; and in some forms it is extremely difficult 

 to say whether a pore exists or not, so nicely graduated are the stages 

 of its development. Consequently, we find genera very closely 

 allied in other characters, differing in this one. Such are Pcecilo- 

 zonites, without pore, Gastrodonta with it; Ariolimax with pore, 

 Prophysaon without it ; and many similar cases. 1 In spite of all the 

 instances of this kind, the tail pore has been a great bugaboo to the 

 systeraatists, while the pedal grooves, of which this pore is merely an 



1 Compare also, in this connection, Cockerell's remarks in Journal of Malacol- 

 ogy (The Conc/iologist), Vol. ii, p. ll'V 



